1. Field of the Invention
The invention relates to a brushless d.c. motor comprising a first motor section, which exhibits circumferentially alternating permanent-magnetic north and south poles adjacent the magnetic air gap, and comprising a second motor section having electrically energizable salient soft-magnetic pole shanks extending towards the magnetic air gap.
2. Description of the Related Art
Motors of the type referred to above are known, for example from DE-U 73 10 863. A problem with such motors is that they exhibit a so-called detent torque. Such a detent torque leads to undesirable mechanical vibrations, noises and speed fluctuations. It is known that the fundamental frequency of the detent torque is given by the product of the mechanical rotational frequency of the motor and the least common multiple of the permanent-magnetic pole pairs of the permanent-magnetic motor section and the number of pole shanks of the electrically energized motor section.
From U.S. Pat. No. 3,860,843 it is known to reduce the detent torque of a brushless d.c. machine in that the pole shoes of the salient soft-magnetic poles of the electrically energized motor section have a more pronounced curvature relative to the air gap than the air gap itself. Conversely, JP-A 60-152240 discloses pole shoes with a pole arc which at the location of the air gap has substantially the same curvature as the air gap but which, in addition, has a salient portion in the pole arc center, which distinctly reduces the effect of the air gap. Another tooth geometry with teeth having salient pole shoes is disclosed in JP-A 60-249838.
In view of the detent torque the prior-art pole shoe geometries are very susceptible to manufacturing tolerances. Very small changes of the geometry already lead to a large change in detent torque. Usually the geometry of the pole shoe arcs is selected in such a way that in general only a Fourier component of the detent torque is reduced strongly. From this point of view the geometry is selected in such a way that the fundamental substantially disappears. However, it is inevitable that the upper harmonics, which in general still lie in a relevant frequency range, still have a significant amplitude.